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Many types of heavy equipment used in the mining and construction industries use enclosed cabs to protect equipment operators from dust and noise exposure. Normally, when the equipment is new, manufacturers' designed controls are effective at keeping the operator's exposure at acceptable levels. However, as equipment becomes older and many components of the enclosure deteriorate, such as gaskets and seals, the effectiveness of the enclosed cab can be greatly reduced. This can cause excessive and dangerous exposures to respirable dust, especially when the ore or overburden contains crystalline silica. In a survey of a number of coal surface mining operations in Pennsylvania a few years ago, high exposure to crystalline silica for surface drill operators was shown to cause an excessive incidence of silicosis. During a recent study to investigate methods to retrofit older cabs with effective control measures, a floor heater in an enclosed cab was documented to be a significant source of high dust exposure to the equipment operator.